This invention relates to an automatic focus servomechanism in an optical information reading device. More particularly, it relates to a focus servomechanism of a convergence lens for converging a bundle of irradiation rays onto an optical information recording medium such as a video disc for use in an optical information reading device of an optical type video disc reproducing system.
In FIGS. 1(a) and 1(b) which are partial plan view of a video disc and a partial cross-sectional view of the video disc, respectively. Reference numeral 6 designates a disc substrate made of transparent materials having on one surface video and/or audio information recorded in the form of pits 17 to form a convolution track or a concentric track. In the video disc, the length of pit and the distance between the pits represent the video and/or audio information. In order to increase reflection efficiency of the video disc, a reflection layer 7 is deposited on one surface of the disc substrate 6 provided with the information patterns in the form of pits 17 for instance by vapor deposition of aluminum. A protection layer 18 is further deposited over the reflection layer 7.
A reading operation of the video and/or audio information recorded on the video disc is carried out by the steps of irradiating the video disc from the other surface of the video disc having no convolution or concentric track, receiving the reflected rays from the reflection layer 7 modulated due to an existance of pit patterns and demodulating the reflected rays to produce the video and/or audio information.
In such an optical information reading device for reading the video and/or audio information recorded on the video disc, a focus servomechanism of a convergence lens has been provided, so that the irradiation rays can be converged to thereby form a fine spot onto the surface of the video disc with high accuracy.
In FIG. 2, a schematic diagram showing the above described focus servomechanism of the convergence lens, a bundle of rays emitted from a light source 1, such as He-Ne laser or the like, passes through a collimating lens 2, a beam splitter 3 and a movable mirror 4. The rays are then converged to form a fine spot in the vicinity of the reflection layers 7 of the video disc by a convergence lens 5. The video disc is rotated by an electric motor 14 at a high rotation speed. The irradiation rays are modulated by the pits 17 to read out the information recorded on the video disc and are reflected by the reflection layer 7, so that the reflected rays follows in the reverse course to the beam splitter 3. The reflected rays are deflected by the beam splitter 3 toward a light receiving element 9 where an optical signal is converted into an electrical signal.
One of the problems in such a system is that it is extremely difficult to shape both surfaces of the disc substrate 6 completely flat. Furthermore, when the video disc is set on a drive shaft of the drive motor 14, there is a possibility that the video disc may be orientated to the drive shaft not in a perpendicular alignment. It is a usual condition that the video disc will not be orientated with respect to the drive shaft in a position plane having the drive shaft perpendicular to that plane. The above difficulties cause the reflection layers 7 to undulate in a vertical direction of the system shown in FIG. 2 according to the rotation of video disc. Accordingly, in order to read out the information recorded on the video disc accurately, it is necessary to instantaneously change the position of the convergence lens 5 in the vertical direction to follow the vertical movement of the reflection surface 7, such that the irradiation rays always converge to thereby form a fine spot in the vicinity of the reflection layer 7. To this end, it has been considered that a cylindrical lens 8 may be disposed on an optical path where the reflected rays by the reflection layer 7 converge by the convergence lens 5. In this case, the cylindrical lens 8 is disposed, as shown in FIG. 2, so that a principle point 20 of the cylindrical lens 8 is located at a convergence point of the reflected rays or in the vicinity thereof.
A light receiving means 9 is disposed behind the cylindrical lens 8 to receive the reflected rays. The light receiving means 9 is made up of four light receiving elements 9a, 9b, 9c and 9d as shown in FIG. 3. The vertical axis passing through the center of the light receiving means 9 coincides with an optical axis of the cylindrical lens 8. As shown in FIG. 3, the light receiving means is arranged in such a manner that it is turned 45.degree. with respect to the center axis thereof. The positional relation between the recording surface 7 and the focus point of the condenser lens 5 is determined by detecting and measuring the cross sectional profile of the light projected onto the four light receiving segments a-d of the light receiving element 9 while utilizing the fact that light portions passing through different portions of the lens are focussed at different points on the optical axis thereof.
The automatic focus servomechanism in the conventional optical information reading device will now be described with reference to FIGS. 2 to 7. The light receiving means 9 is located, so that in the case where the irradiation rays converge to thereby form the fine spot on the recording surface of the video disc accurately, the reflected rays from the convergence point passing through the cylindrical lens 8 form a circular image as shown in FIG. 4(b) on the light receiving surface thereof. Under this condition, the outputs Va, Vb, Vc and Vd of the light receiving elements 9a, 9b, 9c and 9d are equal each other, and therefore the following equation can be obtained: EQU Va+Vb=Vc+Vd
Accordingly, the output V of a differential amplifier 10 to which the sum of Va and Vb and the sum of Vc and Vd are applied as differential inputs, will have a zero output. As a result, no outputs of an amplifier 11 and a convergence lens drive circuit 12 appear at their respective output terminals. Hence, the position of the convergence lens 5 is not shifted.
As shown in FIG. 5, when the irradiation rays are focused at a point in rear of the reflection layer 7 by the convergence lens 5, the image projected through the lenticular lens 8 onto the light receiving surface has a shape shown in FIG. 5(b). This is because the reflection layer 7 approaches the convergence lens 5 due to the undulation of the video disc or the bad setting of the video disc to the drive shaft. Therefore, the sum of the outputs Va and Vb is less than that of the outputs Vc and Vd thereby resulting in the fact that the output V of the differential amplifier 10 is less than zero.
Conversely, as shown in FIG. 6, when the irradiation rays are focused at a point in front of the reflection layer 7 by the convergence lens 5, the image projected through the cylindrical lens 8 onto the light receiving surface has a shape shown in FIG. 6(b). This is because the reflection layer 7 is displaced away from the convergence lens 5. Therefore, the sum of the outputs Va and Vb is larger than that of the outputs Vc and Vd, thereby resulting in the fact that the output V of the differential amplifier 10 is larger than zero.
Assuming that "Z" equals zero in the case where the irradiation rays are focused on the recording surface of video disc, that is, the reflected rays are focused at a principle point 20 or the cylindrical lens as shown in FIG. 4, and further the "Z" is larger than zero in the case where the reflection layer 7 is displaced away from the convergence lens 5, that is, the reflected rays are focused in front of the principle point 20 of the cylindrical lens 8, the output V of the differential amplifier 10 shows a waveform shown in FIG. 7. The output V of the differential amplifier 10 is applied as an error indication signal to the amplifier 11 to be amplified, whereafter the amplified output of the amplifier is applied to the lens drive circuit 12 the operable to change the position of the convergence lens 5 according to the error indication signal. This is accomplished by actuating a lens holder 13 supporting the convergence lens 5. An automatic focus servo operation is carried out in the above described manner.
However, the above-described focus servomechanism is disadvantageous in that it is difficult to always obtain a so-called S-shaped output characteristic curve as shown in FIG. 7 regardless of the position of the light receiving means 9. This is because the shape of image projecting onto the light receiving surface of the light receiving means 9 changes in accordance with the relative position of light receiving element 9 with respect to the cylindrical lens 8. As a result, sufficient automatic focus servo operation is not attained according to such a conventional focus servomechanism.